Electrically semi-conducting engobe coating



July 24, 1962 H. G. SCHURECHT ELECTRICALLY SEMI-CONDUCTING ENGOBE COATING Filed April 21, 1958 2 Sheets-Sheet 1 INVENTOR. Harry 6. (fa/wreck? BY (Q ATTORNEYS Jiily 24, 1962 H. G. SCHURECHT ELECTRICALLY SEML-CONDUCTING ENGOBE COATING Filed April 21, 1958 2 Sheets-Sheet 2 000 com 000 com 000 OOQ SWHO NI HONVLSISBH EISVHHAV ATTORNEYS.

United States Patent 3,046,434 ELECTRICALLY SEMI-CONDUCTING ENGOBE COATING Harry G. Schurecht, Detroit, Mich, assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware Filed Apr. 21, 1958, Ser. No. 729,699 15 Claims. (Cl. 313131) This invention relates to an electrically semi-conducting engobe coating, and, more particularly, to such a coating carried by the nose portion of an insulator of a high energy igniter, and electrically interconnecting the center electrode with the ground electrode thereof.

A recent development in the field of spark plugs and jet engine igniters contemplates a high energy spark discharge over the surface of an electrically semi-conducting ceramic nose of a spark plug or igniter. Although many advantages are achieved from using this typeof discharge to fire reciprocating engines and jet engines, there are also difficulties. When a high voltage is applied to cause the spark, serious problems are encounteredin insulating the ignition system. When only a low voltage is applied the discharge must be along some semi-conducting surface, but, so far as is known, no cornpletely satisfactory electrically semi-conducting material has. heretofore been available for use in the manufacture of such spark plugs or igniters. To be satisfactory for this purpose an electrically semi-conducting engobe material must be capable of production on a relatively large scale with closely reproducible electric characteristics,

and the coatings, when produced, must be capable of withstanding a certain amount of mechanical shock, heat shock incident to the starting and stopping of internal combustion engines, and must be able to withstand for substantial periods of time the erosive atmosphere and temperature conditions prevailing in the firing chamber of an internal combustion engine. In addition, such material must be vitreous, or have a water absorption not greater than about 0.5 percent, so that liquid fuel is not absorbed and carbonized to render the material of too low an electrical resistance. The coating must also be sulficiently refractory to withstand the temperatures encountered in service, and must have a surface electrical resistance of 20,000 ohms to 200,000 ohms when tested in a fixture having a ring in continuous contact with the coating, and 12 fingers, each in electrical contact with the coating at a point spaced A inch from the ring, and with an applied E.M.F. of 500 volts. The preferred resistance when. tested under these conditions is 25,000 to 50,000 ohms.

So far as is known, no completely satisfactory electrically semi-conducting. material has heretofore been available for this use. The problem is made more serious by the tendency of the manufacturers of engines, particularly jet engines, to design for higher and higher combustion chamber operating. temperatures.

The present invention is based upon the discovery that an improved electrically semi-conducting surface can be produced by firing an iron oxide-containing engobe coating. on the nose portion of an insulator. The semi-conducting material so produced can be fired without appreciable detriment, even when embedded in graphite, at temperatures up to about 2000 F. Firing a semi-conducting body embedded in graphite simulates the reducing conditions encountered in, for example, a jet engine combustion zone, and is, therefore, a measure of the ability of the conducting surface to withstand actual opcrating conditions.

It is, therefore, an object of the invention to provide an electrically semi-conducting engobe coating suitable F. produced an insulating coating (resistance in excess of 3,046,434 Patented July 24, 1962 for use on the nose portion of a spark plug or jet engine trically semi-conducting engobe coating fired to vitrifica-,

tion on the nose part of the insulating portion.

Other objects and advantages of the invention will be apparent from the description which follows, reference being had to the accompanying drawings, in which FIG. 1 is a sectional view showing a jet engine igniter according to the invention; and

FIG. 2 is a plot showing dependence upon cooling rate of resistance of a coating according to the invention.

According to the invention anovel electrically semiconducting coating particularly useful as the conducting portion of a high energy spark plug or igniter, and'a method for producing such coating are provided. Such coating is produced by firing a slip in situ on an insulator, and rapidly coolingthe fired assembly. The slip contains from 2 percent to 50 percent of a solid oxide material vitrifiable upon firing, preferably to a temperature from 2400 F. to 3000 F., and from about 98 percent to about 50 percent of iron oxide. The sl'ip'is fired in situ on the insulator to a temperature of at least 2400 F. and preferably from 2400 F. to 3000 F.

The terms percent and parts are usedhe'rein, and in. the appended claims, to refer" to percent and parts by weight, unless otherwise indicated.

Electrically semi-conducting coatings according to the invention have been produced from slips as above defined wherein the iron oxide was FeO, Fe O Fe O4, and an admixture commercially available under the designation black iron'oxide, optimum results having been obtained with thelast-named material. The particular black iron oxide with which optimum results have been achieved is mainly Fe O Other materials which yield iron oxides upon firing can also be used, for example iron carbonate and iron hydroxide. It is preferred-to use from 98 percent to 70 percent of iron oxide, and most preferred to use from percent to percent thereof.

Various solid oxide materials vitrifiable upon firinghave' been. used successfully for producing semi-conducting coatings according to the invention. As examples of such materials, mention may be made" of kaolins generally; feldspar, and various glasses, one of Whichis identified' in' Table I, below. It is preferred to use from 2 per'centto percent to 15 per Aluminum silicates to the invention is critical, as wellas the rate at which.

the coating is cooled from the firing temperature. In this connection, it has been observed experimentally that a firing procedure which produced an electrically'semi-comducting coating when the maturing temperature-was 25 00 megohrns) when the maturing temperature was either 2000 F. or 2200 F. Similarly, when the same coatingwas'fired in a Dressler kiln to a temperature'of 2650 F., with both slow heating to such'ternperaturei and "slow cooling therefrom, the resistance was in excess of 100 megohms. The resistance of the identicalcoating, however, when fired to 2500 F. and then cooled in airfrom such temperature was 150,000 ohms, which. is. a, satisfactory value for such coatings in high energy spark plugs or igniters. Such measurements of resistance were made with a 500 volt megger between two prods separated by Ms inch. Controlled tests have also been run to determine the effect of cooling rate upon electrical resistance of a coating according to the invention, measured as previously described in a fixture having a ring in continuous contact with the coating and twelve spaced fingers each also in electrical contact with the coating. Coating resistance at each of a plurality of initial cooling rates is presented in Table I, below:

cooled at the indicated rate to 1800 F., and in contact with ambient air therebelow; while specimens of Tests 0, D and E were cooled at the indicated rates to 1500 F., and in contact with ambient air therebelow The particular coating tested was identical with that described as body No. 12 in Table II, hereinafter set forth.

The data presented in Table I are represented graphically in FIG. 2 of the drawings. It Will be observed from FIG. 2 that cooling rate has comparatively little effect upon coating resistance so long as the rate is at least as high as 200 F. per minute. Therefore, where it is desired to produce a coating having a resistance, measured as described above, lower than about 100,000 ohms, it is preferred that the cooling rate be at least as high as 200 F. per minute. Since, for spark plug or igniter purposes, a coating having a resistance, measured as described above, not greater than about 200,000 ohms is preferred, a cooling rate of at least about 50 F. per minute should be employed when semi-conducting coatings for use in spark plugs or igniters are being produced,

It will be appreciated that the resistance of an iron oxide-containing engobe coating depends upon the thickness of the coating, as well as upon the cooling rate. By making suitable variations in coating thickness, and by varying cooling rate, it is possible to produce an engobe coating according to the invention having any desired resistance within a relatively broad range. If required, two or more applications of the slip can be made to build up a suflicient body thereof to produce the fired engobe thickness necessary to achieve a particular surface resistance.

In another series of tests, engobe coatings according to the invention, after having been fired and fast-cooled, were reheated slowly to various temperatures, and were then fast-cooled (in ambient air) a second time. The electrical resistance of the coating was then measured. It was found that coatings which had been refired to temperatures from about 1750 F. to about 2300 F. showed substantially increased resistances after refiring. This series of tests demonstrated that a reaction proceeds within a coating according to the invention, at temperatures within the indicated range, and that the reaction product has a high resistance by comparison with a different product which is formed at temperatures as high as 2500 F. Fast cooling of a coating according to the invention should, therefore, be continued from the firing temperature to a temperature not higher than 1750" F., but slow cooling can be employed, if desired from about 1750 F. to room temperature.

The following examples are presented solely for the purpose of further illustrating and disclosing the invention, and are in no way to be construed as limitations thereon.

Example 1 An electrically semi-conducting coating was produced on a ceramic insulator from a slip containing iron oxide and a solid oxide material vitrifiable upon firing to a temperature from 2400 F. to 2750 F. according to the following procedure, which constitutes the best presently known mode for practicing the invention:

A slip was prepared from 87 parts of black iron oxide, 8.7 parts of BF. kaolin, 4.3 parts of feldspar, and 53 parts of water. This slip was applied to the nose portion of a previously fired ceramic insulator containing approximately 92 percent of A1 0 The resulting coated insulator was then placed in a ceramic setter, preheated at about 800 F. and placed in the hot zone of a Globar" furnace at a temperature of about 2300 F. The temperature of the furnace was then raised to about 2500 F., and the coated insulator was immediately removed and allowed to cool in the setter in contact with ambient air. After cooling, the resistance of the fired coating was measured with a 500 volt megger, and was found to be 150,000 ohms. The foregoing firing technique is hereinafter for convenience referred to as Procedure I.

If, for purposes of comparison, but not in accordance with the invention, a procedure identical with that described in the preceding paragraph is repeated except that the maximum firing temperature is either 2000 F. or 2200 F., a measurement with a 500 volt megger indicates that the resistance of the fired coating is in excess of 100 megohms.

A result identical with that described in the first paragraph of this example has been achieved by placing the alumina insulator carrying the coating in a setter in a cold furnace, heating slowly to 2500 F., and then removing the insulator and the fired coating from the furnace and allowing cooling in the setter in contact with ambient air. This firing technique is hereinafter referred to as Procedure II.

Example 2 Various other coatings according to the invention have also been produced. The compositions used, the firing procedure, and the resistance of the fired coating are presented in Table II, below:

TABLE II Firing Resistance In Bod Iron Oxide vitrifiable Material Proceohms of coating No. time as measured by 500 volt megger 3 Magnctite, Kaolin, 8.7 parts; I 75,000

87.0 parts. Feldspar, 4.3

parts. 4 do II 75,000 5 Black iron Kaolin, 10 parts. I 50,000

oxide, 100 parts. 6 Black iron do I 100,000-150,000

oxide, 90 parts. 7 Black iron Kaolin, 20 parts I 75,000-100000 oxide, parts. 8 Black iron Kaolin, 40 parts. I 50,000-200, 000

oxide, 60 parts. 9 F6203, 100 Kaolin, 10 parts; I 500, 000800, 000

parts. Feldspar, 5 parts. 10 R304, 100 do I 80, 000-800,000

parts. 11 Black iron Feldspar, 15 parts... I 50,000

oxide, parts 12 do Feldspar, 10 parts; I 40,000

silica, 5 parts. 13 do Feldspar, 10 parts; I 100,000

Glass, 5 parts. 14 do Feldspar, 5 parts; I 50, 000

Silica, 5 parts; Glass, 5 parts. 15 mos 66 6 TiOz, 32.4 parts. I 1 12,000-39,00

par 3.

See footnote at end of table.

5 TABLE II-Continued Firing Resistance in Body Iron Oxide Vitrifiable Material Proceohms of coating N o. dure as measured by 500 volt megger l6 do T102. 33.4 parts; I 15, 000-120, 000

Whiting, 1.55 parts Talc, 2.07 parts; Bentonite, 1.55 parts. 17 Black iron A120 16 parts; I 40, 000

oxide, 76 Kaolin, 8 parts. parts. 18 Black iron A1 6 parts; I 50,000

oxide, 86 Kaolin, 8 parts. parts. 19 Black iron Feldspar, 10 parts... I 40, 000

oxide, 90 parts. 20 Black iron Kaolin, 10 parts; I 30, 000-50, 000

oxide, 85 Feldspar, parts. parts. 21 (lo Kaolin, 5 parts; I 45, 000-100, 000

Silica, 5 parts; Glass, 5 parts.

for the discharge of a high energy spark. The igniter 11 comprises a metal shell 12 threaded as at 13 for insertion into the combustion chamber, for example, of a jet engine, and threaded at 14 for reception of an ignition harness. A ceramic insulator 15 attached to a second ceramic insulater 16 is sealed inside the metal shell 12, and a center electrode 17 is sealed inside the insulator 15. An electrically semi-conducting coating according to the invention is provided on the nose end of the insulator 15, and is designated by the numeral 18. The coating 18 provides an electrical path interconecting the nose end 19 of the center electrode 17 and a ground electrode 20 attached to the shell 12. When a comparatively low voltage charge is applied to the center electrode of the igniter 11, for example from a condenser, a high energy discharge takes place along the surface of. the semi-conducting coating 18.

The procedure described in Example 1 produced a coating having a thickness of about 0.001 inch. It has been found to be possible to produce coatings as thin as about 0.0005 inch by using an increased proportion of water, or as thick as about 0.005 inch, usually by repeated applications of the slip and firing. In general, the thicker the coating, other factors being equal, the lower the resistance and vice versa. Best results have thus far been achieved when the engobe thickness has been from about 0.001 inch to albout 0.003 inch.

It has also been found that the adherence between an engobe coating according to the invention and an insulator for example of the composition identified in Example 1 can be increased by use of a suitable glaze. For

example, a glaze slip has been prepared from 65 parts I of water and 35 parts of a composition having the following analysis:

Percent Kaolin 7.4 Feldspar 26.2 Silica flint 18.2 Whiting 13.1 White lead 12.6 ZnO 3.7 Frit 18.8 Bentonite 0.2

The frit used had the following analysis:

Percent NA O 1.36 K 0 173 CaO 4 17 The proportion of water in the slip identified in the preceding paragraph is comparatively high in order to produce a thin glaze after application of the slip to the insulator and firing. It has been found that such a thin glaze is essential, when used as an engobe undercoating, in order to achieve optimum adherence without blistering of the latter. When engobe No. 12, identified in Example 2, above, was applied over such a glaze, which had been fired to 2425 F., the resulting assembly was found to meet the most rigorous of testing procedures.

in order to be satisfactory for producing an engobe under-glaze which will increase adherence, as indicated above, a glaze slip should contain from about 45 percent to about 85 percent of watenpreferably from about 55 percent to'about 75 percent of water. Such a slip produces a glaze, when applied by dipping, in the vicinity of 0.0005 inch to 0.001 inch in thickness.

The adherence of an engobe according to the inven tion to a ceramic insulator can also be improved by applying a slip made from copper metal powder, a vitrifiable material and Waterto the insulator and firing, before application and firing of one. or'more engo be coatings. Excellent results have been achieved Where the first slip included 95 parts of copper metalpowder and 5 parts of gaolin and where the slip was fired to 2200 F.; two coatings of body No; 12, each. fired to 2400 F., were then used.

This is a continuation-in-part of application Serial No.

476,379, filed December 20, 1954, now abandoned, entitled Electrically Semi-Conducting Engobe Coating.

- it will be apparent that various changes and modifications can be made from the specific details disclosed and described without departing from the spirit of the attached claims.

What I claim is:

l. A method for producing an electrically 'semi-conducting coating which comprises applying to a surface of v a ceramic insulator, a slip 'containingfrom 2 percent to 30 percent ocf'a' solid oxide material vitrifiable upon. firing to a temperature from 2400 F. to 3000 F., and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a tempera-ture not higher than about 1750 F. and at a-rate'of at least 50 F. per minute. 1

2. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 2' per'cent'to 30 percent of aluminum silicates vitrifialb'le upon firing to a temperature from 2400 F. to 2750 F., and from about 98 percent to about 70 percent of iron oxide, heatingthe resulting assembly to. a temperature from 2400" F. to"

2750" F., and rapidly cooling the fired assembly to a temperature not higher than about 1750" F. and at a rate of at least 50 F. per minute.

3. A method for producing an electrically'semi-conducting coating which comprises applying to a surface o a ceramic insulator, a slip containing from 10 percent to 15 percent-of aluminum silicates vitrifia'ble upon firing to a temperature from 2400 F. to 2750" F., and from about 90 percent to 85 percent of iron oxide, heating the resulting assembly to a temperature from 2400 F. to 2750 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750 F. and at a rate of at least 50 F. per minute.

4. A method for producing an electrically semi-conducting coating which comprises applying a slip to a surface of a ceramic insulator, said slip containing from about 70 percent to about 98 percent of iron oxide and from 30 percent to 2 percent of a solid oxide material vitrifiable upon firing to a temperature from 2400 F. to 27 50 F., heating the resulting assembly to a temperature from 2400 F. to-2750 F., and cooling the fired assembly from the firing temperature in ambient air to a temperature not higher than about 1750 F.

5. A method'for producing an electrically semi-com ducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 2 percent to 50 percent of a solid oxide material vitrifiable upon firing to a temperature from 2400 F. to 2750 F., and from about 98 percent to about 50 percent of iron oxide, heating the resulting assembly to a temperature from 2400 F. to 2750" F., and rapidly cooling the fired assembly to a temperature not higher than about 1750 F. and at a rate of at least 50 F. per minute.

6. A spark plug comprising at least two spaced metal electrodes and an electrically semi-conducting coating produced by the method claimed in claim 5 mechanically and electrically connecting said electrodes, the coating having a surface electrical resistance not greater than about 200,000 ohms.

7. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly in ambient air to a temperature not higher than about 1750 F.

8. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 15 percent to 55 percent of a glaze composition and from 85 percent to 45 percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip contain- ;ing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly in ambient air to a temperature not' higher than about 1750" F.

9. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 25 percent to 45 percent of a glaze composition and from 75 percent to 5 5 percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750 F. and at a rate of at least 50 F. per minute.

10. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing 35 percent of a glaze composition and 65 percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750" F. and at a rate of at least 50 F. per minute.

11. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 15 percent to 55 percent of a glaze composition comprising feldspar, silica, whiting and white lead and from 85 percent to percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly in ambient air to a temperature not higher than about 1750" F.

12. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing to a temperature from 2400 F. to 3000 F., and from about 98 percent to 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750" F. and at a rate of at least F. per minute.

13. A method for producing an electrically semi-com ducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing to a temperature from 2400 F. to 3000 F., and from about 98 percent to 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F.,

and rapidly cooling the fired assembly to a temperature not higher than about 1750 F. and at a rate of at least 200 F. per minute.

14. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 15 percent to percent of a glaze composition and from 85 percent to 45 percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750 F., and at a rate of at least 50 F. per minute.

15. A method for producing an electrically semi-conducting coating which comprises applying to a surface of a ceramic insulator, a slip containing from 15 percent to 55 percent of a glaze composition and from percent to 45 percent of water, firing the coated insulator to mature the glaze, applying over the matured glaze a slip containing from 2 percent to 30 percent of a solid oxide material vitrifiable upon firing, and from about 98 percent to about 70 percent of iron oxide, heating the resulting assembly to a temperature of at least 2400 F., and rapidly cooling the fired assembly to a temperature not higher than about 1750 F., and at a rate of at least 200 F. per minute.

Smits Dec. 18, 1951 Sanborn Apr. 1, 1952 

1. A METHOD FOR PRODUCING AN ELECTRICALLY SEMI-CONDUCTING COATING WHICH COMPRISES APPLYING TO A SURFACE OF A CERAMIC INSULATOR, A SLIP CONTAINING FROM 2 PERCENT TO 30 PERCENT OF A SOLID OXIDE MATERIAL VITRIFIABLE UPON FIRING 